The present invention relates to the general field of inline exhaust fan assemblies, and more particularly to mixed flow fan assemblies.
In a mixed flow fan assembly, the primary exhaust gas/air flow enters the impeller axially, i.e., parallel to the impeller shaft axis, and is discharged from the impeller with both axial and radial velocity components. The objective of the present invention is to provide a mixed flow fan assembly with greater static efficiency and reduced noise output, thereby reducing the energy required to run the fan at an equivalent performance level. The fan assembly described herein is designed to operate upstream of a discharge nozzle, such as the induction nozzle described in U.S. patent application Ser. No. 13/067,269, the disclosure of which is incorporated herein by reference.
Because of the geometry of prior art designs, specifically the relatively large fan wheel (impeller) shroud angle with respect to a vertical reference line, impeller blade positioning, inlet bell design and positioning, and impeller offsets, discharging primary airflow (i.e. air that enters through the inlet bell) will recirculate through the fan wheel. The air is processed through the fan wheel, and as it discharges the fan wheel, a portion of the total primary flow recirculates back through the impeller offset (between the inlet bell and the impeller shroud) to be reprocessed by the fan wheel (impeller). A separate portion of the primary flow continues in suspended rotation in the space between the exterior of the inlet bell and the interior of the fan housing in the direction of impeller rotation. This recirculation reduces efficiency by reducing the total flow capacity of the impeller by the portion of airflow that is recirculated by the impeller.
Moreover, in the annular exhaust plenum relatively short axial guide vanes are employed that are typically mounted a substantial distance from the impeller discharge. While there must be adequate space between the bottom of the guide vanes and the trailing edge of the impeller blades to allow the airflow to develop as it discharges the rotating impeller, too much separation between the guide vanes and the impeller discharge leads to highly rotational flow and the development of vortices/turbulence in the annular exhaust plenum, which consumes available energy and reduces overall efficiency.